In the underground power distribution industry, elastomeric elbows and bushings have seen more than a decade of commercial usage as separable connector elements. Such elbows typically comprise housings with an electrically stress-graded end interfittable with a shielded power cable and an opposite end having an elongate cylindrical contact assembly electrically connected to the cable conductor and receivable by a female contact in the bushing. The bushing contact is in turn electrically connected to user apparatus, for example, a transformer or the like. In adapting the elbow-bushing separable connector to usage in electric arcing situations, i.e., loadmake, loadbreak and fault closure conditions, the elbow contact assembly is generally comprised of an electrically conductive contact (rod) and a rod extension (follower) of material adapted to generate arc-extinguishing gases upon being exposed to electric arcing. In turn, the bushing female contact is combined with a block of like arc-extinguishing material.
For safety in the joinder and separation of elbows and bushings under energized circuit conditions, the industry has adopted the so-called "hot-stick" technique, whereby an operator engages the elbow by use of an elongate stick of some ten foot length the thereby moves the elbow into or away from the bushing. With such distance involved, it is unavoidable that occasions arise wherein there is substantial cantilever stressing of the composite rod and rod extension, i.e., where the hot-stick is not axially in alignment with the bushing female contact element. The rod, being of metal, readily accommodates such cantilever stressing. On the other hand, the rod extension, being constituted of non-metallic arc-quenching material, has quite limited resistance to cantilever stress and has been observed to exhibit cracking. In lessening cracking of arc-quenching material upon cantilever stressing thereof, the industry has in the past reinforced the arc-quenching material by running a rigid extension of the rod interiorly of the arc-quenching material for a portion of its length. In these initial embodiments, the art provided such improved cantilever stress resistance by running a rigid electrically conductive (metal pin) member from the male contact to a location axially interior of the extremity of the arc-quenching material, thereby also providing electrical stress relief for the interface of the rod and rod follower.
In a more recent development, set forth in U.S. Pat. No. 3,955,874, it is proposed that the foregoing metal pin member practice is not adequate in that the follower remains susceptible to breakage in its extent axially beyond the pin member. In accommodating its proposed solution to the problem, the effort in such patent provides a solid electrically insulative member of rigid nature extending the full length of the rod follower and includes, for purposes of stress relief, an electrically conductive film on the exterior of such rigid insulative member extending less than the extent of the follower.
In arranging its electrically conductive film directly interiorly of the arc-quenching material, the U.S. Pat. No. 3,955,874 contact assembly exposes such film to electric arcing upon the existence of fissures in the arc-quenching material. A direct electrical path for arcing current flow is accordingly provided to the male contact through the exposed electrically conductive film. Additionally, in this arrangement contact assembly volume at and adjacent the end face of the follower directly exposed to arcing is occupied by the solid electrically insulative member which is coterminus with the follower at such end face.